1. Field of the Invention
The invention relates to a power supply apparatus for a vehicle, which includes a main power supply that supplies electric power to a plurality of vehicle-mounted electric loads, and a subsidiary power supply charged with the electric power output from the main power supply, and a method of controlling the power supply apparatus.
2. Description of the Related Art
For example, an electric power steering apparatus generally includes an electric motor that provides steering assist torque to the turning operation of a steering wheel. The steering assist torque is adjusted by controlling the feeding of electric power to the electric motor. Such an electric power steering apparatus uses a vehicle-mounted battery as a power source, and consumes large electric power. Therefore, for example, an apparatus proposed in Japanese Patent Application Publication No. 2007-91122 (JP-A-2007-91122) includes a subsidiary power supply that assists the vehicle-mounted battery. The subsidiary power supply is connected in parallel to a power supply line that extends from the vehicle-mounted battery (hereinafter, referred to as “main power supply”) to a motor drive circuit. The subsidiary power supply is charged with the electric power output from the main power supply. The subsidiary power supply supplies the electric power to the motor drive circuit using the electric power with which the subsidiary power supply is charged. The apparatus proposed in the publication No. 2007-91122 includes a switch that allows and prohibits the feeding of electric power from the subsidiary power supply to the motor drive circuit; and a switch that allows and prohibits charging of the subsidiary power supply with the electric power output from the main power supply.
However, in the apparatus proposed in the publication No. 2007-91122, when a target value of electric power used to operate the electric motor exceeds a threshold value, the switch is simply turned on to form a power supply circuit that connects the subsidiary power supply to the motor drive circuit. Therefore, it is not possible to appropriately supply the electric power from the subsidiary power supply to the motor drive circuit. That is, because the subsidiary power supply assists the supply of the electric power according to balance between the voltage of the main power supply and the voltage of the subsidiary power supply, sufficient electric power may not be supplied from the subsidiary power supply to the motor drive circuit until the voltage of the main power supply is greatly decreased.
Therefore, when the electric power steering apparatus is operated using a large output, the subsidiary power supply assists the supply of the electric power to the motor drive circuit late. As a result, the voltage of the main power supply is greatly decreased, and a vehicle-mounted lighting device, such as a headlight, flickers, and accordingly, a drive feels discomfort.